The invention relates to holey fibres and other analogous cavity waveguiding structures.
A holey fibre is an optical fibre whose optical confinement mechanism and properties are affected by an array of air holes defined by cavities that run lengthwise down the fibre. Light can be guided within holey fibres by two distinct mechanisms. First, with periodic arrangements of air holes, guidance can be obtained through photonic band gap effects [1]. Second, guidance can be obtained from volume average refractive index effects. This second guidance mechanism does not rely on periodicity of the air holes [2].
Generally, a holey fibre has a solid core surrounded by a holey cladding region. The structure is fabricated by stacking silica capillaries in a hexagonal close packed array and then removing one of the capillaries and replacing it with a solid silica rod of the same outer dimensions. The fibre stack is then drawn to bundle form in a one or two stage process. Fabrication of holey fibres is discussed further in the literature [3][4]. In the literature, the core is sometimes referred to as the central index defect, or defect for short, and the surrounding holey cladding as the hole lattice.
Holey fibres are generally characterised in terms of hole size and hole spacing. Because of the fabrication method, the holes are usually periodically spaced, the period usually being termed as xe2x80x9cpitchxe2x80x9d, xcex9, in the literature. Because of the fabrication method, the holes are also usually circular and thus categorised by a diameter, d. Typical dimensions of existing holey fibres are at least 2 microns for the pitch, although some smaller pitches are mentioned in the literature. Specifically, reference [2] discloses a pitch xcex9=0.925 xcexcm with hole diameters of d=0.688 or 0.833 xcexcm. Moreover, reference [7] discusses a silica core of diameter d=0.962 xcexcm suspended in air in the context of a theoretical study related to using holey fibre to compensate for group velocity dispersion in standard telecommunications fibre.
Further general background relating to theories applicable to holey fibres may be found in the literature [5][6].
One application suggested for holey fibres is sensing. It is proposed that a fluid, i.e. gas or liquid, is present in the fibre cavities. A property of the fluid is then sensed by its effect on that part of the optical mode, generally an evanescent wave part, which propagates in the holey cladding region.
Another application suggested for holey fibres is for low-loss telecommunication fibre. In principal, it might be expected that propagation losses could be reduced in a holey fibre, by virtue of the lower losses associated with the holes relative to the glass regions of the fibre. More fundamentally, a holey fibre with a photonic band gap could reduce losses through photonic crystal effects.
These applications pre-suppose that a significant proportion of mode power is present in the holey cladding region of the fibre. Holey fibres of the type based on photonic band gap guidance mechanism have very recently been fabricated with a large percentage of mode power in the xe2x80x9cactivexe2x80x9d holes of the cladding region [14]. However, this type of holey fibre requires periodicity in the hole structure and thus can be expected to be quite difficult to fabricate commercially. Consequently, holey fibres of the type based on the average refractive index guiding mechanism are in principal more attractive since there is no equivalent requirement of hole periodicity. However, examples of this latter type of holey fibre fabricated to date have only a very small percentage of mode power in the xe2x80x9cactivexe2x80x9d holes of the cladding region. This fact, not hitherto appreciated, has been determined by a theoretical model developed by the inventors, as described in references [8] and [9]. The full contents of references [8] and [9] are incorporated herein by reference.
Reference [8] describes a scalar orthogonal function method for holey fibres, which is valid when the holes are small.
Reference [9] extends the model to the vector case, which enables the full range of holey fibres to be modelled. This technique involves decomposing the modal field using localised functions. The central index defect and the hole lattice are described independently using localised functions for the defect and periodic functions for the holes. This can be efficient and accurate because the quantities are described by functions chosen carefully to suit.
The model of references [8] and [9] allows computation of the proportion of mode power present in the holes of the cladding region of a holey fibre. Using this model, it has been determined that previously fabricated holey fibres guiding by average refractive index effects typically have only around 1% or less of their mode power in the holes.
Clearly, this is not good for any of the proposed holey fibre devices whose efficiency depends on mode power in the holes.
It is therefore an aim of the invention to provide a holey fibre based on average refractive index guidance effects which has a relatively high proportion of mode power in the holes, i.e. a relatively large evanescent overlap of the mode field with the fibre cavities.
According to a first aspect of the invention there is provided a holey fibre having holes with a pitch of less than 0.9 microns, more preferably approximately 0.75 microns, positioned adjacent to the core. The pitch is preferably between 0.1 and 0.9 microns, more preferably between 0.5 and 0.9 microns. The ratio of hole size to pitch is preferably greater than or equal to approximately 0.6.
According to a second aspect of the invention there is provided a method of guiding light along an optical fibre by average refractive index effects, the optical fibre having a core and a cladding, the cladding containing holes distributed across the optical fibre to define a pitch, wherein the wavelength of the light is at least approximately 2.2 times the pitch of the holes. The holes preferably have a pitch of less than 0.9 microns, more preferably approximately 0.75 microns. The ratio of hole size to pitch is preferably greater than or equal to approximately 0.6. In an embodiment of the invention, the holes are approximately circular and the ratio of hole diameter to pitch is between 0.6 and 0.8.
With these aspects of the invention it is possible to achieve a massive improvement in the mode power present in the holes without photonic band gap effects. The percentage fraction of fundamental mode power located in the holes is used as a figure of merit. In the prior art holey fibres using average refractive index guidance effects, this is generally less than 1%, often much less. With the invention, it is possible to realise holey fibres based on average refractive index guidance effects that have 10-40% or more of the fundamental mode power in the holes. In some embodiments, the holey fibre may have a highly periodic hole structure and thus may possess significant photonic band gap effects.
The design rules specified above thus allow for large evanescent overlap of the mode field with the air, other fluid or vacuum present in the fibre holes. The holes should be arranged adjacent to the core so that they interact significantly with the optical mode guided by the core. In addition to the small pitch holes arranged around the core, there may be a farther group of holes radially outward of the small pitch holes, for example larger holes for ease of fabrication. On the other hand, the small pitch holes may extend across all the cladding region and constitute the only group of holes in the cladding region.
The proposed holey fibres generally incorporate relatively large amounts of air within the structure, typically with a space (air) fill factor in the cladding of greater than 40%. Moreover, the hole spacing, i.e. pitch, should preferably be shorter or comparable to the optical wavelength of the mode of interest.
The invention will be of potential interest to all applications requiring an optical interaction with a liquid, gaseous or vacuum field by evanescent field effects. For example, the concentration of pollutants in a gas could be determined by measuring the absorption which occurs as light propagates through the gas for a range of wavelengths [10]. Particular applications of interest are:
1) transport of high power optical beams (low optical non-linearity fibre);
2) low-loss optical fibre for transmission systems;
3) optical sensors (gas detection, liquid composition, medical);
4) atom optics;
5) optical manipulation of microscopic particles;
6) particle separation (by mass, induced polarisability, electric dipole moment);
7) Raman lasers;
8) non-linear optical devices;
9) referencing of a laser to specific gas absorption lines;
10) metrology; and
11) dispersion compensation in transmission systems (holey fibre embodying the invention can be made to exhibit high dispersion).